Availability

Embargo Period

Degree Type

Degree Name

Department

Date of Defense

First Committee Member

David S. Nolan

Second Committee Member

Bruce A. Albrecht

Third Committee Member

Brian E. Mapes

Fourth Committee Member

Robert F. Rogers

Fifth Committee Member

Hugh E. Willoughby

Abstract

The dynamics and evolution of spiral rainbands in numerical simulations of tropical cyclones are examined. Two types of numerical simulations of tropical cyclones that are popular in recent literature are considered: a realistic numerical simulation of Hurricane Bill (2009) that includes most physical processes that occur within tropical cyclones, and an idealized, no-mean-flow numerical simulation of a tropical cyclone. First, spiral rainbands in the numerical simulation of Hurricane Bill are examined. There appears to be four types of spiral rainbands that occur in Hurricane Bill: principal, secondary, distant, and inner rainbands. Principal rainbands tilt radially outward with height, and dry air marks their radially outward boundary. The structures of secondary rainbands are in good agreement with those from previous observational studies, except their convective-scale structure. Distant rainbands tilt radially inward with height and are a form of density currents. Inner rainbands are made of shallow convection. Previous studies have argued that inner rainbands are the manifestation of either gravity or vortex-Rossby waves. However, the propagation of these inner rainbands in the Hurricane Bill is found to be inconsistent with those previous hypotheses because they are not consistently collocated with pressure or potential vorticity anomalies that are expected from gravity or vortex-Rossby waves, respectively. It is not consistent with tropical squall lines either, because surface cold pools and mechanical lifting are not collocated with inner rainbands. A different hypothesis is proposed to explain the propagation of inner rainbands, which views inner rainbands as blobs of convection that are advected by the low-level horizontal wind field while being deformed into spiral shapes. Examining the evolution of spiral rainbands in the idealized, no-mean-flow simulation of a tropical cyclone indicates that they are only two types of spiral rainbands: distant and inner rainbands. Distant rainbands behave as density currents, and inner rainbands are again found to be inconsistent with vortex-Rossby waves. Since the no-mean-flow simulation contains only distant and inner rainbands and the effects of principal and secondary rainbands on tropical cyclones are not yet known, the framework of the idealized no-mean-flow simulations of tropical cyclones may not be the best suited to study spiral rainbands.